Ultimately it is a redundancy question. As mammals, we have a need for air-like gas to breath, even while we are scuba diving. Just like the spare tire in the trunk of your car, some level of redundancy is a good idea for our breathing supply while diving.
There are three major components to a regulator system; a supply of gas, a first stage of pressure reduction to reduce the varying tank level to a consistent intermediate pressure, and a second stage for final pressure reduction to a breathable level. When we add an octo to our regulator system, we now have a redundant second stage, yet we still share the gas supply and first stage. If we use an H-valve, then we have upped the redundancy to now have two first and two second stages, with the only shared component being the gas supply. Finally, if we add a pony bottle or isolatable doubles to the mix, now we have true total redundancy, with completely separate breathing sources.
It's a risk management question. Shallow dives, with a buddy, no overhead restrictions or deco obligations, an octo should be sufficient to deal with any issues that come up, including a total failure of your or your buddy's breathing source, such as an out of air situation, tank neck o-ring failure or burst disk rupture, or the failure of the first or second stage.
Adding an H-valve lets you address the failure of either a first or second stage, by letting you shut off the failed system and still retaining gas in the tank to be delivered by the other first//second stage system. Of course, that redundancy also includes practice and the ability to reach back, locate and close the tank valve on the failed side before the tank empties itself. And of course, losing the tank o-ring or burst disk still results in an empty tank.
Finally, having that separate pony bottle, or an isolation manifold on your doubles, gives you the greatest level of protection in the event of any sort of failure in your breathing supply. Of course, the diver must be able to react, and reach back, find and shut off the isolation valve in a set of double to accomplish this. A pony bottle set-up, on the other hand, requires no action at all on the divers part except switching the regulator in your mouth.
And correspondingly, as you add more gear, you add cost, complexity and a greater maintenance requirement, but that should all be part of yoru risk assesment and management plan.
Hope this helps.
I forget that once you go tech life changes. It is no longer a pony but a deco bottle. It's no longer a tank but a cylinder. You no longer go OOA but OOG (actually you had better have a good reason for OOG). And most important, when diving the US/Canada waters you have to know which side is which.. BTW, the brown waters are Canadian. Right Steve? 
